Methods and systems for guarding a charge transfer capacitance sensor for proximity detection

ABSTRACT

Methods, systems and devices are described for determining a measurable capacitance for proximity detection in a sensor having a plurality of sensing electrodes and at least one guarding electrode. A charge transfer process is executed for at least two executions. The charge transfer process includes applying a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, applying a first guard voltage to the at least one guarding electrode using a second switch, sharing charge between the at least one of the plurality of sensing electrodes and a filter capacitance, and applying a second guard voltage different from the first guard voltage to the at least one guarding electrode. A voltage is measured on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.

PRIORITY DATA

This application is a continuation application of U.S. patentapplication Ser. No. 11/833,828, filed Aug. 3, 2007, which is acontinuation application of U.S. patent application Ser. No. 11/445,856,which was filed on Jun. 3, 2006, which issued on Aug. 28, 2007, as U.S.Pat. No. 7,262,609, which claims priority to U.S. Provisional PatentApplication Ser. Nos. 60/687,012; 60/687,148; 60/687,167; 60/687,039;and 60/687,037, which were filed on Jun. 3, 2005 and Ser. No. 60/774,843which was filed on Feb. 16, 2006, and are incorporated herein byreference.

TECHNICAL FIELD

The present invention generally relates to capacitance sensing, and moreparticularly relates to devices, systems and methods capable ofdetecting a measurable capacitance using switched charge transfertechniques.

BACKGROUND

Capacitance sensors/sensing systems that respond to charge, current, orvoltage can be used to detect position or proximity (or motion orpresence or any similar information), and are commonly used as inputdevices for computers, personal digital assistants (PDAs), media playersand recorders, video game players, consumer electronics, cellularphones, payphones, point-of-sale terminals, automatic teller machines,kiosks, and the like. Capacitive sensing techniques are used inapplications such as user input buttons, slide controls, scroll rings,scroll strips, and other types of inputs and controls. One type ofcapacitance sensor used in such applications is the button-type sensor,which can be used to provide information about the proximity or presenceof an input. Another type of capacitance sensor used in suchapplications is the touchpad-type sensor, which can be used to provideinformation about an input such as the position, motion, and/or similarinformation along one axis (1-D sensor), two axes (2-D sensor), or moreaxes. Both the button-type and touchpad-type sensors can also optionallybe configured to provide additional information such as some indicationof the force, duration, or amount of capacitive coupling associated withthe input. Examples of 1-D and 2-D touchpad-type sensor based oncapacitive sensing technologies are described in United States PublishedApplication 2004/0252109 A1 to Trent et al. and U.S. Pat. No. 5,880,411,which issued to Gillespie et al. on Mar. 9, 1999. Such 1-D and 2-Dsensors can be readily found, for example, in input devices ofelectronic systems including handheld and notebook-type computers.

A user generally operates a capacitive input device by placing or movingone or more fingers, styli, and/or objects, near the input device an ina sensing region of one or more sensors located on or in the inputdevice. This creates a capacitive effect upon a carrier signal appliedto the sensing region that can be detected and correlated to positionalinformation (such as the position(s), proximity, motion(s), and/orsimilar information) of the stimulus/stimuli with respect to the sensingregion. This positional information can in turn be used to select, move,scroll, or manipulate any combination of text, graphics, cursors,highlighters, and/or any other indicator on a display screen. Thispositional information can also be used to enable the user to interactwith an interface, such as to control volume, to adjust brightness, orto achieve any other purpose.

Although capacitance sensors have been widely adopted, sensor designerscontinue to look for ways to improve the sensors' functionality andeffectiveness. In particular, engineers continually strive to reduce theeffects of spurious noise on such sensors. Many capacitive sensors, forexample, currently include ground planes or other structures that shieldthe sensing regions from external and internal noise signals. Whileground planes and other types of shields held at a roughly constantvoltage can effectively prevent some spurious signals from interferingwith sensor operation, they can also reduce sensor resolution orincrease parasitic effects, such as by increasing parasitic capacitance.Therefore, the performance of such devices is by no means ideal.

Accordingly, it is desirable to provide systems and methods for quickly,effectively and efficiently detecting a measurable capacitance whilepreventing at least some of the adverse effects that can result fromspurious noise signals and/or enhance resolution. Moreover, it isdesirable to create a scheme that can be implemented using readilyavailable components, such as standard ICs, microcontrollers, andpassive components. Other desirable features and characteristics willbecome apparent from the subsequent detailed description and theappended claims, taken in conjunction with the accompanying drawings andthe foregoing technical field and background.

BRIEF SUMMARY

Methods, systems and devices are described for determining a measurablecapacitance for proximity detection in a sensor having a plurality ofsensing electrodes and at least one guarding electrode. A chargetransfer process is executed for at least two executions. The chargetransfer process includes applying a pre-determined voltage to at leastone of the plurality of sensing electrodes using a first switch,applying a first guard voltage to the at least one guarding electrodeusing a second switch, sharing charge between the at least one of theplurality of sensing electrodes and a filter capacitance, and applying asecond guard voltage different from the first guard voltage to the atleast one guarding electrode. A voltage is measured on the filtercapacitance for a number of measurements equal to at least one toproduce at least one result to determine the measurable capacitance forproximity detection.

Using the techniques described herein, a guarded capacitance detectionscheme may be conveniently implemented using readily availablecomponents, and can be particularly useful in sensing the position of afinger, stylus or other object with respect to a capacitive sensorimplementing button, slider, cursor control, or user interfacenavigation functions, or any other functions.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention will hereinafter be describedin conjunction with the following drawing figures, wherein like numeralsdenote like elements, and:

FIG. 1A is a flowchart of an exemplary technique for detectingcapacitance using switched charge transfer techniques with guarding;

FIG. 1B is a block diagram of an exemplary capacitive proximity sensorthat includes guard circuitry;

FIG. 1C is a timing diagram relating to an exemplary technique foroperating the capacitive proximity sensor with guard circuitry of FIG.1B;

FIGS. 2A-B are timing diagrams of exemplary guard signals that can beapplied to guarding electrodes.

FIGS. 3A-E are block diagrams of exemplary circuits that could be usedto generate guard voltages of a guard signal;

FIGS. 4A-E are more detailed block diagrams of exemplary circuits thatcould be used to generate guard voltages of a guard signal; and

FIG. 5 is a schematic diagram of a proximity sensor device with anelectronic system.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

According to various exemplary embodiments, a capacitance detectionand/or measurement circuit can be readily formulated using two or moreswitches. Further, a guard signal with two or more guarding voltages canbe applied to a guarding electrode using one or more additional switchesand one or more passive electrical networks (which can be a simple wireor a complex network); this can be used to shield the sensor fromundesired electrical coupling, thereby improving sensor performance. Ina typical implementation, a charge transfer process is executed for twoor more iterations. In the charge transfer process, a pre-determinedvoltage is applied to a measurable capacitance using one or more of theswitches and a first guarding voltage is applied to a guarding electrodewith a second switch, the measurable capacitance then shares charge witha filter capacitance in the passive network and a second guardingvoltage is applied to the guarding electrode. With such a chargetransfer process, a plurality of applications of the pre-determinedvoltage and the associated sharings of charge influence the voltage onthe filter capacitance. The voltage on the filter capacitance can be thevoltage at a node of the circuit that indicates the voltage across thefilter capacitance. The voltage on the filter capacitance can also bethe voltage across the filter capacitance itself. The charge transferprocess thus can be considered to roughly “integrate” charge onto thefilter capacitance over multiple executions such that the “output”voltage of the filter capacitance is filtered. The charge transferprocess may be done using only switches and passive elements such asresistances, capacitances, and/or inductances. After one or moreiterations of the charge transfer process, the voltage on the filtercapacitance (which is representative of the charge on the filtercapacitance) is measured. One or more measurings can be used to produceone or more results and to determine the measurable capacitance. Themeasuring of the voltage on the filter capacitance can be as simple as acomparison of the voltage on the filter capacitance with a thresholdvoltage, or be as complex as a multi-step analog-to-digital conversionextracting charge from the filter capacitance and measuring the voltagemultiple times. Using these techniques, capacitive position sensorscapable of detecting the presence or proximity of a finger, stylus, orother object can be readily formulated. Additionally, variousembodiments of the guard described herein can be readily implementedusing only conventional switching mechanisms (e.g. signal pins of acontrol device) and passive components (e.g. one or more capacitors,resistors, inductors and/or the like), without the need for additionalactive electronics that would add cost and complexity. The variousguarding techniques described herein can use similar components andmethods as charge transfer sensing techniques. This, coupled with theease of multi-channel integration, provide for highly efficientimplementation of the guard. As a result, the various guarding schemes(and sensing schemes if desired) described herein may be convenientlyyet reliably implemented in a variety of environments usingreadily-available and reasonably-priced components, as described morefully below.

With reference now to FIG. 1A, an exemplary technique 800 for detectinga measurable capacitance that provides guarding to shield the measurablecapacitance from undesired electrical coupling is illustrated. Themethod 800 uses switched charge transfer to detect measurablecapacitances, and is particularly applicable to the detection ofcapacitances for object position detection. The technique suitablyincludes the broad steps of performing a charge transfer process withvoltage guarding (step 801) for two or more times (as repeated by step810) and selectively measuring a voltage on the filter capacitance toproduce a result (step 824). The charge transfer process 801 includesapplying a pre-determined voltage to the measurable capacitance (step802). Then, a first guard voltage is applied to a guarding electrode(step 804). The first guard voltage is preferably provided before theapplying of the pre-determined voltage to the measurable capacitanceceases. Then, charge is shared by the measurable capacitance and afilter capacitance (step 806). “Sharing” charge in this context canrefer to actively switching to couple the measurable capacitance and thefilter capacitance, actively switching elsewhere in the system,otherwise directing the transfer of charge, or passively allowing thecharge to transfer through impedance through quiescence or otherinaction. Then, a second guard voltage is applied to the guardingelectrode (step 808). The second guard voltage is different from thefirst guard voltage, and is preferably applied to the guarding electrodebefore the sharing of charge substantially ends. The charge transferprocess repeats at least once (step 810) for at least two performancesof the charge transfer process total, and may repeat many more times.The charge transfer process can repeat until the voltage on filtercapacitance exceeds a threshold voltage, until the process 801 hasexecuted for a pre-determined number of times, and/or according to anyother scheme. Each time the charge transfer process executes, the firstand second guard voltages are provided to shield from undesirableelectrical coupling.

Measurement of the voltage on the filter capacitance to produce a result(step 824) can take place at any time, including before, after, andduring the charge transfer process. In addition, none, one, or multiplemeasurements of the voltage on the filter capacitance 824 can be takenfor each repetition such that the number of measurement results to thenumber of charge transfer processes performed can be of any ratio,including one-to-many, one-to-one, and many-to-one. Preferably thevoltage on filter capacitance is measured when the voltage on the filtercapacitance is substantially constant. One or more of the measurementresults is/are used in a determination of the value of the measurablecapacitance. The value of the measurable capacitance may take placeaccording to any technique. In various embodiments, the determination ismade based upon the measurement(s) of the voltage on the filtercapacitance (which is indicative of the charge on the filtercapacitance), the values of known components in the system (e.g. thefilter capacitance), as well as the number of times that the chargetransfer process 801 was performed. As noted just previously, theparticular number of times that process 801 is performed may bedetermined according to a pre-determined value, according to the voltageacross the filter capacitance crossing a threshold voltage, or any otherfactor as appropriate.

Steps 802-808 and steps 824 can be repeated as needed (step 810). Forexample, in a proximity sensor implementation, the measurablecapacitance corresponding to each sensing electrode would typically bedetermined many times per second. This provides the ability to determinethe proximity of objects near the sensor, as well as changes to thatproximity, and thus facilitates use of the process in a device for userinput. Thus, the process can be repeated at a high rate for each sensingelectrode each second to enable many determinations of the measurablecapacitance per second.

Process 800 may be executed in any manner. In various embodiments,process 800 is executed by software or firmware residing in a digitalmemory, such as a memory located within or in communication with acontroller, or any other digital storage medium (e.g. optical ormagnetic disk, modulated signal transmitted on a carrier wave, and/orthe like). Process 800 and its various equivalents and derivativesdiscussed above can also be executed with any type of programmedcircuitry or other logic as appropriate.

The steps of applying first and second guard voltages can be implementedwith a variety of different techniques and devices. For example, theguard voltages can be provided using switching mechanisms and passivecomponents (e.g. one or more capacitors, resistors, inductors, and/orthe like), without the need for additional active electronics that wouldadd cost and complexity (although such active electronics, includingDACs and followers, can be used to provide the proper guard voltages atlow impedance).

Now with initial reference to FIG. 1B, an exemplary capacitance sensor100 suitably includes three sensing electrodes 112A-C and one guardingelectrode 106. The sensing electrodes 112A-C are directly coupled toswitches 116A-C, respectively. The sensing electrodes 112A-C are alsodirectly coupled with a filter capacitance (also “integratingcapacitance” or “integrating filter”) 110 (CF) through passiveimpedances 108A-C, respectively. The filter capacitance 110 is alsoshown directly coupled to a switch 118. The guarding electrode 106 iscoupled to a guarding voltage generating circuit 104 that includespassive guarding network 105 and one or more switch(es) 114. Guardingvoltage generating circuit 104 provides an appropriate guard signal (VG)103. Also shown in FIG. 1B is stimulus 101 that is not part ofcapacitance sensor 100 and is detected by capacitance sensor 100.Stimulus 101 can be one or more fingers, styli, objects, and the like,even though one stylus is shown in FIG. 1B.

Although a specific configuration of sensor 100 is shown in FIG. 1B, itis understood that many other configurations are possible. Otherembodiments of capacitance sensor 100 may include any number of sensingelectrodes, guarding electrode, filter capacitances, passive impedances,switches, guarding voltage generating circuits, and controllers asappropriate for the sensor. They can also be in any ratio appropriatefor the sensor; for example, the sensing electrodes may also be coupledto filter capacitance(s) with or without passive impedances in amany-to-one, one-to-many, one-to-one, or many-to-many configuration asallowable by the sensing scheme used. It should be noted that while FIG.1B shows switch(es) 114, 116A-C, and 118 all implemented using I/Os of acontroller 102, that this is just one example embodiment, and that theseand other switches could be implemented with a variety of differentdevices including discrete switches distinct from any controller. Asfurther examples, the sensor may use a passive guarding network thatconsists of a single wire or a more complex circuit network, or thesensor may also provide the guarding signal using a single switch ormultiple switches (which may involve using one or many I/Os of acontroller, a multiplexer, a digital-to-analog converter (DAC), etc.,since each multiplexer or DAC includes multiple switches). A switch canbe used in a multitude of ways to provide the guard signal, includingclosing the switch, opening the switch, or actuating it in some othermanner (e.g. PWM and pulse coded modulating). Therefore, one can apply avoltage by closing a switch as well as by opening a switch, depending onhow the circuit is laid out. Additional analog components may also beused (e.g. to buffer the output of the passive guarding network 105).

The sensing electrodes 112A-C provide the measurable capacitances whosevalues are indicative of the changes in the electric field associatedwith stimulus 101. Each of the measurable capacitances represents theeffective capacitance of the associated sensing electrode(s) 112A-Cdetectable by the capacitance sensor 100. In an “absolute capacitance”detecting scheme, the measurable capacitance represents the totaleffective capacitance from a sensing electrode to the local ground ofthe system. In a “trans-capacitance” detection scheme, the measurablecapacitance represents the total effective capacitance between thesensing electrode and one or more driving electrodes. Thus, the totaleffective capacitance can be quite complex, involving capacitances,resistances, and inductances in series and in parallel as defined by thesensor design and the operating environment. However, in many cases themeasurable capacitance from the input can be modeled simply as a smallvariable capacitance in parallel with a fixed background capacitance.

To determine the measurable capacitances, appropriate voltage signalsare applied to the various electrodes 106, 112A-C using any number ofswitches 114, 116A-C. In various embodiments, the operation of switches114, 116A-C is controlled by a controller 102 (which can be amicroprocessor or any other controller). By applying proper signalsusing switches 116A-C, the measurable capacitances exhibited byelectrodes 112A-C (respectively) can be determined. Moreover, byapplying proper signals using switch(es) 114, suitable guarding voltagescan be generated to produce a guard signal 103 that is placed onguarding electrode 106 to shield the measurable capacitances fromundesired effects of noise and other spurious signals during operationof sensor 100.

Guarding electrode 106 is any structure capable of exhibiting appliedguarding voltages comprising guard signal 103 to prevent undesiredcapacitive coupling with one or more measurable capacitances. AlthoughFIG. 1B shows guarding electrode 106 with a “comb”-type appearance, thisappearance is shown for convenience of explanation, and guardingelectrode 106 may exhibit any other form or shape, in any number ofequivalent embodiments as applicable for the design of sensor 100. Forexample, the sensing electrodes 112A-C may be laid out in some otherpattern or have some other shape, and the shape of guarding electrode106 can be laid out as appropriate. Guarding electrode 106 can also berouted around all or portions of a perimeter of a set of sensingelectrodes to shield the set at least partially from the environment.Guarding electrode 106 can be routed behind at least a portion of thesensing electrodes to shield them from any electronics behind thesensing electrodes. Guarding electrode 106 can also be routed betweensensing electrodes to shield them from each other. The guardingelectrode does not need to extend the full length between sensingelectrodes or cover the full sensing electrodes to offer a useful levelof guarding. For example, guarding electrode 106 can parallel onlyportions of the sensing electrodes 112A-C, or interleave some or all ofthe sensing electrodes 112A-C. In addition, if a “trans-capacitance”detection scheme is used, guarding electrode 106 may be routed aroundany areas where guarding electrode 106 may interfere with the capacitivecoupling between the sensing electrodes 112A-C and any drivingelectrode(s), such as some regions between the sensing electrodes 112A-Cand the driving electrode(s). As explained below, capacitive couplingbetween guarding electrode 106 and measurable capacitances can becontrolled through application of appropriate guarding voltages viaswitch(es) 114.

In the exemplary embodiment shown in FIG. 1B, a filter capacitance 110is provided by one or more capacitors (such as any number of discretecapacitors) to accept charge transferred from sensing electrodes 112A-C.Although the particular filter capacitance value selected will vary fromembodiment to embodiment, the capacitance of each filter capacitance 110will typically be much greater—perhaps by only one to two orders ofmagnitude but often several orders of magnitude greater—than thecapacitance of the measurable capacitances. Filter capacitance 110 maybe designed to be on the order of several nanofarads, for example, whenexpected values of measurable capacitances are on the order of severalpicofarads or so. Actual values of filter capacitance 110 may vary,however, depending upon the particular embodiment.

The concepts of capacitance sensing in conjunction with guarding can beapplied across a wide array of sensor architectures 100, although aparticular example is shown in FIG. 1B. In the exemplary embodimentshown in FIG. 1B, each sensing electrode 112A-C, and thus eachassociated measurable capacitance, is coupled to a common filtercapacitance 110 through an associated passive impedance 108A-C.Alternate embodiments may use multiple filter capacitances and/orpassive impedances for each measurable capacitance as appropriate.Alternate embodiments may also share a passive impedance and/or a filtercapacitance between multiple measurable capacitances. When included,passive impedances 108A-C are typically provided by one or morenon-active electronic components, such as any type of diodes,capacitors, inductors, resistors, and/or the like. Passive impedances108A-C are each generally designed to have an impedance that is largeenough to prevent significant current bleeding into filter capacitance110 during charging of measurable capacitance, as described more fullybelow. In various embodiments, impedances 108A-C may be on the order ofa hundred kilo-ohms or more, although other embodiments may utilizewidely different impedance values. Again, however, passive impedances108A-C need not be present in all embodiments where charge sharing isotherwise implemented.

Operation of sensor 100 suitably involves a charge transfer process anda measurement process facilitated by the use of one or more switches116A-C, 118 while a guard signal 103 is applied using switch(es) 114.Again, although shown implemented using I/Os of controller 102, switches114, 116A-C and/or 118 may be implemented with any type of discreteswitches, multiplexers, field effect transistors and/or other switchingconstructs, to name just a few examples. Alternatively, any of switches114, 116A-C, 118 can be implemented with internal logic/circuitrycoupled to an output pin or input/output (I/O) pin of the controller102, as shown in FIG. 1B. Such I/O pins, if used, can also provide inputfunctionality and/or additional switches. For example, switch 118 can beimplemented with I/O 119 that also connects to, or contains, inputcapability within controller 102. The input capability may be used inmeasuring the voltage on the filter capacitance 110 directly orindirectly, and might include a multiplexer, comparator, hystereticthresholds, CMOS threshold, or analog-to-digital converter. Such I/Opins are typically capable of switchably applying one or more logicvalues and/or a “high impedance” or “open circuit” value by usinginternal switches coupled to power supply voltages. The logic values maybe any appropriate voltages or other signals. For example, a logic“high” or “1” value could correspond to a “high” voltage (e.g. 5 volts),and a logic “low” or “0” value could correspond to a comparatively “low”voltage (e.g. local system ground, −5 volts or the like). The particularsignals selected and applied can vary significantly from implementationto implementation depending on the particular controller 102, sensorconfiguration, and sensing scheme selected. For example, a currentsource, a pull-up resistance, or a digital-to-analog converter (DAC)also could be used to provide the proper voltages, and may be externalor internal to controller 102.

One advantage of many embodiments is that a very versatile capacitancesensor 100 can be readily implemented using only passive components inconjunction with a controller 102 that is a conventional digitalcontroller comprised of any combination of one or more microcontrollers,digital signal processors, microprocessors, programmable logic arrays,integrated circuits, other controller circuitry, and/or the like. Anumber of these controller products are readily available from variouscommercial sources including Microchip Technologies of Chandler, Ariz.;Freescale Semiconductor of Austin, Tex.; and Texas Instruments ofRichardson, Tex., among others. Controller 102 can contain digitalmemory (e.g. static, dynamic or flash random access memory) that can beused to store data and instructions used to execute the various chargetransfer processing routines for the various capacitance sensorscontained herein. During operation of various embodiments, the onlyelectrical actuation on the sensing electrodes 112A-C and theirassociated measurable capacitances that need take place during operationof sensor 100 involves manipulation of switches 114, 116A-C and 118;such manipulation may take place in response to configuration, software,firmware, or other instructions contained within controller 102.

The charge transfer process, which is typically repeated two or moretimes, suitably involves using a first switch to apply a pre-determinedvoltage (such as a power supply voltage, battery voltage, ground, orlogic signal) to charge the applicable measurable capacitance(s), andthen passively or actively allowing the applicable measurablecapacitance(s) to share charge with any filter capacitance (e.g. 110) asappropriate. Passive sharing can be achieved by charge transfer throughan impedance such as a resistance, and active sharing can be achieved byactivating a switch that couples the applicable measurablecapacitance(s) to the appropriate filter capacitance(s).

The pre-determined voltage is often a single convenient voltage, such asa power supply voltage, a battery voltage, a digital logic level, aresistance driven by a current source, a divided or amplified version ofany of these voltages, and the like. The value of the pre-determinedvoltage is often known, and often remains constant; however, neitherneeds be the case so long as the pre-determined voltage remainsratiometric with the measurement of the voltage on the applicable filtercapacitance (e.g. 110). For example, a capacitance sensing scheme caninvolve resetting the filter capacitance to a reset voltage, and alsoinvolve measuring a voltage on the filter capacitance by comparing thevoltage (as relative to the reset voltage) on one side of the filtercapacitance with a threshold voltage (also as relative to the resetvoltage). With such a sensing scheme, the difference between thepre-determined voltage and the reset voltage, and the difference betweenthe threshold voltage and the reset voltage, should remain roughlyproportional to each other, on average over the executions of the chargetransfer process leading to the determination of the measurablecapacitance. Thus, the threshold used to measure the change in voltageon the filter capacitance will be proportional to the change in voltageon the filter capacitance due to the charge shared from the measurablecapacitance to the filter capacitance during the executions of thecharge transfer process for a determination of the measurablecapacitance. In particular, where the pre-determined voltage is V_(dd)and the reset voltage is GND, the threshold voltage can be ratiometricfor a CMOS input threshold, for example (½)*(V_(dd)−GND).

The example shown in FIG. 1B can be operated in a manner as shown byFIG. 1C. In the embodiment shown by FIGS. 1B-C, each switch 116A-Capplies a pre-determined voltage with “charging pulses” 201 thattypically have relatively short periods in comparison to the RC timeconstants of impedances 108A-C with the filter capacitance 110, andpreferably have relatively short periods in comparison to the RC timeconstants of impedances 108A-C with their associated measurablecapacitances. This is so that the charge added to filter capacitance 110during the charge transfer process comes mostly from the charge storedon the active measurable capacitance and shared with filter capacitance110, and less from any flow of current through the associated impedance(e.g. 108A-C) during the applying of the pre-determined voltage. Thishelps to prevent excessive leakage of current through impedances 108A-C.Also shown in FIG. 1C, each charging pulse 201 additionally providesrelatively brief durations of an “opposing” “discharging voltage” (avoltage that have a magnitude opposite that of the pre-determinedvoltage) before applying the pre-determined voltage. The dischargingvoltage can compensate for any current leaking through impedances 108A-Cduring the charge transfer process; it is an optional feature that isnot required in all embodiments. More than one level of voltage can beused in the pre-determined voltage in an execution or betweenexecutions, and this is also true for the opposing voltage. However, inmany cases the pre-determined voltage and the opposing voltage (if used)will have substantially constant voltages.

The following discussion describes the operation with one guardingelectrode (e.g. 106), one measurable capacitance (e.g. associated withsensing electrodes 112A-C), one filter capacitance 110, and often onepassive impedance (e.g. 108A-C). This is done for clarity ofexplanation, and it is understood that multiple measurable capacitances,passive impedances, and filter capacitances can be included in thesystem, and they can be operated in serially (at least partially orcompletely separate in time) or in parallel (at least partially orcompletely overlapping in time ).

After applying the pre-determined voltage to the measurable capacitance,the measurable capacitance is allowed to share charge with filtercapacitance. To allow measurable capacitance to share charge, no actionmay be required other than to stop applying the pre-determined voltageand pause for a time sufficient to allow charge to passively transfer.In various embodiments, the pause time may be relatively short (e.g. ifthe filter capacitance is connected directly to the measurablecapacitance with a small resistance in series), or some delay time mayoccur (e.g. for charge to transfer through a larger resistance in serieswith the measurable capacitance, the filter capacitance, and referencevoltage). In other embodiments, allowing charge to transfer may involvestopping the application of the pre-determined voltage and activelyactuating one or more switches associated with a controller to couplethe measurable capacitance and the filter capacitance, and/or takingother actions as appropriate. For example, charge sharing with thefilter capacitance could occur in other embodiments using “sigma-delta”techniques; such as in a process whereby the filter capacitance ischarged via a measurable capacitance and discharged by a “delta”capacitance (not shown), or vice versa. As another example, chargesharing with the filter capacitance could occur by actuating switches(not shown) that couple and decouple the measurable capacitance with thefilter capacitance or that couple and decouple the filter capacitancewith a power supply voltage. In such embodiments, impedances such asthose shown as 108A-C shown in FIG. 1B may not be present, may beaugmented by passive or active elements, and/or may be replaced bypassive or active elements as appropriate.

A charge transfer process where sharing charge between the measurablecapacitance and the filter capacitance occurs using one or more activecomponents (e.g. by actively opening or closing a switch) clearlyindicates the beginning and the end of a sharing period with theseactuations of the active component(s). Similarly, a charge transferprocess where the measurable capacitance is directly connected to oneside of the filter capacitance, and the other side of the filtercapacitance is coupled, by activating a switch, to a low impedancereference voltage, also clearly indicates the beginning and ending of asharing period. In contrast, charge transfer processes that passivelyshare charge have less clear denotations of the charge sharing periods.In the systems that passively share charge, the charge sharing periodcan be considered to begin when the applying of the pre-determinedvoltage ceases; the charge sharing period must end at or before asubsequent charging pulse begins (for a subsequent execution of thecharge transfer process) and at or before a reset of the filtercapacitance (if a reset is used and indicates an end a set of chargetransfer processes). The sharing period may end before a subsequentcharging pulse and before any reset because current flow effectivelystops when the voltages are similar enough that negligible charge isshared between the measurable capacitance and the filter capacitance;this will be the case when sufficient time has passed while themeasurable capacitance and filter capacitance are coupled to each other.However, even if the voltages do not substantially equalize before asubsequent charging pulse or reset signal, charge sharing still endswhen the charging pulse or reset signal begins. This is because theapplying of the charging pulse or reset signal dominates over any chargesharing between the measurable capacitance and the filter capacitance ina passive sharing system where the filter capacitance is always coupledto the measurable capacitance (such as in sensor 100 of FIG. 1B). Thelow impedance path of the charging pulse or reset signal means that anycharge on the measurable capacitance that would be shared with thefilter capacitance is negligible until the low impedance source isremoved.

The measurement process may be performed at any point of the chargetransfer process as appropriate for the sensor configuration and sensingscheme used, and the number of performances of the measurement processmay be in any ratio with the performances of the charge transfer processas appropriate for the sensor configuration and sensing scheme used. Forexample, the measurement process may take place after the sharing of thecharge between the measurable capacitance and the filter capacitancebrings the voltage on the filter capacitance to be within somepercentage point from an asymptote, or the measurement process may takeplace every time a charge transfer process is performed. Conversely, themeasurement process may take place while the pre-determined voltage isapplied (if the filter capacitance is properly prevented from chargesharing with the measurable capacitance at that time). The measurementprocess may take place only for a set number of repetitions of thecharge transfer process, or only after a number of repetitions havealready taken place. The measuring of the voltage on the filtercapacitance can be as simple as a comparison of a voltage on the filtercapacitance with a threshold voltage (such as in a “sigma-delta”scheme), or be as complex as a multi-step analog-to-digital conversion(such as when a known number of charge transfer processes are performedand then the voltage on the filter capacitance is read as a multi-bitvalue). Multiple thresholds can also be used, such as in an oscillatoror other dual-slope sensing system where the voltage on the filtercapacitance is driven between low and high thresholds, and in multi-bitADCs where multiple thresholds are used to measure the voltage on thefilter capacitance. One or more measurements can be taken, and stored ifappropriate, to determine the measurable capacitance as applicable.

More detail about particular capacitance sensing schemes can be found invarious literature, in U.S. Pat. Nos. 5,730,165, 6,466,036, and6,323,846, as well as in U.S. patent applications entitled Methods andSystems for Detecting a Capacitance Using Switched Charge Transfertechniques, by David Ely et al, filed Jun. 3, 2006 and Methods andSystems for Detecting a Capacitance Using Sigma-Delta MeasurementTechniques, by Kirk Hargreaves et al, filed Jun. 3, 2006. Again, theparticular capacitance sensing technique and sensor architecture 100 mayvary significantly in other embodiments.

A system without any shields or guards will be affected by theenvironment. Therefore, as discussed earlier, many capacitive sensorsinclude ground planes or other structures that shield the sensingregions from external and internal noise signals. However, ground planesand other types of shields held at a roughly constant voltage are by nomeans ideal—they can increase the effects of parasitic capacitance (orother parasitic impedance and associated charge leakage) and reduceresolution or dynamic range. In contrast, a driven, low-impedance guardcan provide similar shielding without significantly increasing theeffect of parasitic capacitance or reducing resolution. This is done byreducing the charge transferred through any parasitic capacitancesassociated with any guarding electrode(s) onto any filter capacitance(s)during the course of executions of the charge transfer processes leadingto the determination of the measurable capacitance(s). The voltages ofthe guard can be provided by using an output from a charge transferprocess similar to the one to be guarded. This output can be provided asan input to a buffer (or other follower circuit) to guard multiplesensing channels with low impedance. Alternatively, these guard voltagescan also be directly provided by using a guard-charge transfer process(one performed for guarding purposes) that inherently provides a lowimpedance guard signal such that no additional buffering is needed; thisguard-charge transfer process could also be similar to the chargetransfer process used for sensing, but that is not required.

The typical charge transfer sensing scheme will perform the chargetransfer processes multiple times (and often hundreds of times or more)to generate the measurement(s) that are used for one determination ofthe measurable capacitance. This set of charge transfer processes thatlead to the measurement(s) used for one determination varies betweenembodiments. As four examples, the set can be between a reset state anda final-threshold-state for systems that charge to threshold(s); the setcan be between an initial state and a final-read-state for systems thatperform a set number of charge transfer processes and read one or moremulti-bit voltage output(s); the set can be between the low and highthresholds for dual slope or oscillator systems; the set can also be thesample length of a digital filter for sigma-delta systems. This set ofcharge transfer processes defines a set where the overall guardingeffect is considered, or “the course of executions of the chargetransfer processes leading to the determination of the measurablecapacitance.”

To reduce the net charge transferred through the parasitic capacitanceassociated with the guarding electrode onto the filter capacitanceduring the course of executions of the charge transfer processes leadingto the determination of the measurable capacitance(s), a guard signalwith proper guarding voltages can be applied. The applying of thepre-determined charging voltage to the measurable capacitance lasts forsome duration of time, and before this duration ends, a first guardingvoltage similar to this pre-determined voltage can be applied to theappropriate guarding electrode. Since the pre-determined voltage istypically fairly constant, the first guarding voltage can often be asingle, roughly constant voltage. Then, before all the charge is shared(i.e., before charge sharing ends) between the measurable capacitanceand associated filter capacitance, the guard signal applied to theguarding electrode may be changed to a second guarding voltage similarto the voltage on the associated filter capacitance. Again, although thesingular is used in this discussion, there can be any number of guardingelectrodes, measurable capacitances, impedances, filter capacitances,and the like involved.

In the embodiment shown in FIG. 1B, guarding electrode 106 is provided,over a low impedance path, with guarding voltages composing guard signal103 that at least roughly approximate the voltages on the activeelectrode (e.g. 112A-C) during the sensing process. If a fairly constantpre-determined voltage is applied to charge the measurable capacitances,the guard signal 103 that is applied to the guarding electrode 106before the applying of the pre-determined voltage is finished cancomprise a single voltage similar to this pre-determined voltage. Then,before the charge transfer between the measurable capacitance withfilter capacitance 110 ends (i.e. before the sharing period ends), theguard signal 103 applied to the guarding electrode 106 may be changed toa guarding voltage similar to the voltage on filter capacitance 110. Ifthe guard signal 103 is changed to a second guarding voltage that is asubstantially constant voltage during the charge sharing period and formultiple executions of the charge transfer process, it can be a voltagechosen to approximate the voltage on the filter capacitance 110.Approximations are appropriate when discrete voltages are used in guardsignal 103, since the voltage on the filter capacitance 110 changesduring sharing and between repetitions of the charge transfer process.For example, the guarding voltage of the guard signal 103 applied toguarding electrode 106 may be set to the pre-determined voltage duringthe charging of the measurable capacitance, and then changed from thepre-determined voltage to a voltage between an appropriate thresholdvoltage (VTH) and an after-reset-voltage on the associated filtercapacitance 110 to reduce the net transfer of charge. Any DC offsetbetween the guarding electrode voltage and the sensing electrode voltagewould not affect the usefulness of the guard for capacitive coupling,since similar voltage swing (i.e. similar change in voltage) istypically of greater concern than the actual voltage applied, inensuring an effective guard.

The guarding voltages of guard signal (VG) 103 may be generated in anymanner. Even though the embodiment shown in FIGS. 1B-C describes guardsignal 103 as being generated by an I/O that enables switch(es) 114 toapply power supply voltages, it is understood that many otherembodiments are possible. For example, alternate sources for guardsignal 103 may involve discrete switches, multiplexers, operationalamplifiers (OP-AMPs), follower, or ADCs other than digital I/Os, utilizecurrent and/or voltage sources, and may be separate from the controllerimplementing the charge transfer process. In addition, digital-to-analogconverters, pulse-width modulators, and the like can also be used togenerate the guard signals 103 in various equivalent embodiments. Inaddition, a wide range of voltages different from that used by thecharge transfer process can be applied. For example, the voltage sourcefor guard signal 103 (if a voltage source and not a current or someother source is used), and even the guarding voltages of guard signal103 themselves, may be beyond the range defined by the pre-determinedvoltage and the filter capacitance reset voltage. It is also understoodthat one or multiple guard signals may be used in systems havingmultiple guarding electrodes 106. In addition, sensing electrodes may beused as guarding electrodes when they are “inactive” in not being usedto sense.

In the embodiment shown in FIG. 1B-C, guarding electrode 106 isconnected to an appropriate guarding voltage generating circuit 104.Guard voltage generating circuit 104 appropriately includes one or moreswitch(es) 114, which is implemented as an I/O of controller 102.Circuit 104 is any suitable circuitry capable of producing two or moredifferent values of voltages on guarding electrode 106 in response to asignal applied by switch(es) 114, although particular examples of guardvoltage generating circuit 104 are described below (e.g. in conjunctionwith FIGS. 3A-E and 4A-E). In various embodiments, passive guardingnetwork 105 of circuit 104 is implemented with conventional passiveimpedance circuitry (such as a voltage or impedance dividing circuit)including one or more conventional resistors, inductors, and/orcapacitors. Passive guarding network 105 is shown directly connected toguarding electrode 106 in sensor 100; in other implementations,switches, followers, or other elements may intervene.

In one embodiment, guard signal 103 includes voltages that areapproximately equal to voltages associated with the charge transferprocess. Guard signal 103 includes an “approximate-charging-voltage”that approximates the pre-determined voltage applied to any “active”sensing electrode(s) to charge them during the charging period (e.g. oneor more of the sensing electrodes 112A-C associated with measurablecapacitances). Guard signal 103 also includes an“approximate-sharing-voltage” that approximately equals the voltageassociated with any “active” sensing electrodes being shared with thefilter capacitance 110 during the sharing period when charge sharing isallowed. In this embodiment, the guarding signal 103 begins applying theapproximate-charging-voltage to the guarding electrode 106 before theapplying of the pre-determined voltage ends (i.e. before the chargingperiod terminates). The approximate-charging-voltage can be applied atother times as well, such as during the entire charging period or duringother portions of the charging period. There is flexibility in when toapply the approximate-charging-voltage since the active sensingelectrodes (e.g. 112A-C) are driven during that period, and any effectsof parasitic capacitances coupling guarding electrode 106 to the activesensing electrodes would be negligible. The guard signal 103 changes tobegin applying the approximate-sharing-voltage to the guarding electrode106 before the end of the sharing of the charge between any activesensing electrode (e.g. 112A-C) with the associated filter capacitance110. Similar to the applying of the approximate-charging-voltage, thereis flexibility in when to begin applying theapproximate-sharing-voltage. For example, this applying of theapproximate-sharing-voltage can take place during the entire duration ofthe period when charge is allowed to be shared between the activesensing electrodes (e.g. 112A-C) or only near the end of the period. Forthe guard to be effective, it should typically provide a relatively lowimpedance when applying these two approximate guarding voltages.However, the guard need not always be driven with a low impedance whennot applying these two guarding voltages, though its effectiveness as aguard may be reduced.

The general sensor and guard scheme described above and shown in FIG. 1Bcan be supplemented or modified in many different ways. In variousembodiments, an included capacitance (not shown) can be included inguard voltage generating circuit 104 to temporarily store charge removedfrom the various sensing channels associated with sensing electrodes112A-C. This charge can be returned to the appropriate sensing channel(often back to the electrodes 112A-C themselves) during subsequentoperation. Stated another way, by maintaining the charge on the includedcapacitance at a relatively constant value (e.g. through application ofelectrical signals using switch(es) 114), the net amount of chargeshared between the filter capacitance 110 and the included capacitancethrough the sensing electrodes 112A-C can be reduced. Typically, theincluded capacitance is designed to be much larger (at least an order ofmagnitude or greater) relative to the particular total capacitance ofguarded capacitances between sensing electrodes 112A-C and guardingelectrode 106, and often larger than the associated filter capacitance110. In such embodiments, the low impedance guarding signal 103 isrelatively immune to coupling effects from sensing electrodes 112A-C andany other electrodes due to the much larger included capacitance. As aresult, a single guarding electrode 106 may be used to effectivelyshield multiple sensing electrodes 112A-C from both undesirable internaland external coupling, including the coupling from one sensing channelto another, if the sensing scheme warrants. As shown, guarding signal103 may be of low impedance and effective even when the switch(es) usedto generate the guarding voltages are open. Many other enhancements oralterations could be made in addition to those described herein. Forexample, the output of a guard voltage generating circuit 104 might beactively buffered to provide a guard to multiple sensing electrodes, ifthe output is of high impedance.

With reference to FIG. 1C, an exemplary timing scheme 150 is shown thatwould be suitable for operating sensor 100 of FIG. 1B using a “switchedRC time-constant” manner of charge transfer sensing. The particulartiming scheme 150 shown in FIG. 1C applies predominantly to sensing ofmeasurable capacitance of sensing electrode 112A. Similar processeswould be executed to measure charge on the electrodes associated withmeasurable capacitances of sensing electrodes 112B-C as well. It shouldbe noted that in cases where the various measurable capacitances share acommon filter capacitance 110, the sensing channels associated with thesensing electrodes would typically be operated in sequence and notsimultaneously for this particular example. However, parallel operationcould take place in an equivalent embodiment such as one in which eachmeasurable capacitance was provided with its own filter capacitance 110,or such as one in which a coded or frequency modulated sequence wasapplied to individual sensing channels.

During the “switched RC time-constant” sensing process shown in timingscheme 150, the measurable capacitance associated with sensing electrode112A is provided with charging voltage pulses 201 using switch 116A. Inthis embodiment, switch 116A is implemented using a digital I/O ofcontroller 102. Since a digital I/O can typically provide logic high andlow voltages (e.g. V_(dd), and GND), it is simple to apply a chargevoltage pulse having the pre-determined voltage of V_(dd). Betweenprovisions of charging pulses 201, the measurable capacitance associatedwith sensing electrode 112A is allowed to discharge into filtercapacitance 110 via passive impedance 108A. This is noted by the voltagetraces for V_(x) 117A (corresponding to the voltage on the measurablecapacitance associated with sensing electrode 112A at the node coupledto switch 116A) and V_(F) 115 (corresponding to the voltage on filtercapacitance 110 at the node coupled to I/O 119). V_(x) 117A rises to thepre-determined voltage (e.g. V_(dd)) when the pre-determined voltage isapplied during the charging period, and then decreases with the timeconstant defined by the measurable capacitance associated with sensingelectrode 112A and passive impedance 108A during the charge sharingperiod when the measurable capacitance discharges into filtercapacitance 110. Meanwhile, the voltage on filter capacitance 110 slowlyincreases as it is charged by the measurable capacitance associated withsensing electrode 112A during the sharing period. During the sharingperiod, V_(x) 117A and V_(F) 115 approach the same value, since the tworespective capacitances are sharing charge. In most embodiments, thesharing period will be set long enough to enable V_(x) 117A and V_(F)115 to share enough charge such that they are essentially the same bythe end of the sharing period. This makes the system less sensitive totiming variations.

Between a previous sharing period and a subsequent charging period, anoptional “current canceling” voltage is applied to the measurablecapacitance. The timing of the “current canceling” voltage is controlledso the amount of “parasitic” charge removed from the filter capacitance110 is mostly equal to the amount of “parasitic” charge added to filtercapacitance 110 through passive impedance 108A during the chargingperiod, and the measurable capacitance is still left at the propercharging voltage before sharing with the filter capacitance 110. Thismay allow for a lower value for passive impedance 108A, and faster timeconstants as a whole without changing the measurable capacitance chargetiming requirements.

The input/output pin 119 of controller 102 that provides switch 118 alsomeasures the voltage 115 on the filter capacitance. The I/O 119 suitablycontains or connects to a comparator (which is a one-bit quantizer thatcan be used to provide a signal bit analog-to-digital conversion),Schmitt trigger, CMOS threshold, and/or multi-bit analog-to-digitalconverter feature that is capable of measuring voltage V_(F) 115 atvarious times (e.g. 202A-C) when switch 118 is open. When a comparatoris used to measure the voltage 115, the V_(TH) can be made roughlyequivalent to the midpoint between the high and low logic values tosimplify the system. V_(TH) is roughly the midpoint between the high andlow logic values with a simple exemplary CMOS threshold.

In the particular embodiment shown in FIGS. 1C, the measurablecapacitance associated with sensing electrode 112A is charged anddischarged until the voltage V_(F) 115 on filter capacitance 110 exceedsa threshold voltage V_(TH) associated with I/O 119. As I/O 119 sensesthat the threshold voltage V_(TH) has been passed (indicated by point202C), a reset signal 203 is provided using switch 118 of I/O 119.Switch 118 applies the reset signal 203 which resets the chargecontained on filter capacitance 110 after voltage V_(F) 115 exceeds athreshold voltage V_(TH). FIG. 1C shows the “reading” of I/O 119 tomeasure the voltage on filter capacitance 110 immediately after asharing period and starting only after some repetitions of the chargetransfer process have already taken place (after a resetting of filtercapacitance 110). However, as discussed earlier, other timing andfrequency options exist for measuring the voltage on filter capacitance110 and are contemplated here. For example, additional charge transferprocesses could be performed and/or additional measurements made aftervoltage V_(F) 115 exceeds a threshold voltage V_(TH).

By tracking the number of charge transfer cycles performed from theapplying of the reset signal 203 until the voltage on filter capacitance110 exceeds the threshold voltage V_(TH), the measurable capacitance canbe effectively determined. That is, the number of repetitions of thecharge transfer process performed to produce a known amount of charge onfilter capacitance 110 (e.g. as indicated by the voltage at the measurednode of the filter capacitance reaching V_(TH)) can be effectivelycorrelated to the actual capacitance of the measurable capacitance.Similarly, the number of oscillations or resets of the filtercapacitance 110 occurring for a number of the charge transfer processescan also be used to determine the measurable capacitance.

The embodiment shown in FIGS. 1B-C shows the reset signal 203 resettingfilter capacitance 110 by setting the voltage on the node of the filtercapacitance 110 coupled to switch 118 to local system ground, such thatboth sides of the filter capacitance are set at ground. This can be seenin the trace V_(F) 115 dropping to V_(RESET) in response to the resetsignal 203. In other embodiments, resetting of filter capacitance 110can be accomplished in a wide variety of ways, and the options availabledepend on the sensor configuration and sensing scheme chosen. In variousembodiments, a reset signal 203 can be used to set one side of thefilter capacitance 110, or the voltage across filter capacitance 110, toan appropriate reset voltage appropriate for the sensing. Resetting offilter capacitance 110 can also be accomplished by simply coupling aswitch on one side of the filter capacitance 110 to the appropriatepower supply voltage. Alternatively, where both sides of filtercapacitance 110 are controlled by switches, the voltage on the filtercapacitance 110 may be reset to a pre-determined value by applying knownvoltages on both sides of the filter capacitance 110. In addition,filter capacitance 110 can comprise a network of capacitors instead ofone single capacitor, and each capacitor in the network may be reset toa different voltage and controlled by one or more switches, such thatresetting filter capacitance 110 may involve opening and closing amultitude of switches.

Reset signal 203 may be provided periodically, aperiodically, orotherwise, and/or may not be provided at all in some embodiments to“reset” the sensor. However, such systems would still exhibit what maybe considered a “reset voltage” for guarding purposes. For example,other embodiments utilizing RC networks do not have an equivalent ofswitch 118 (shown in FIG. 1B) for active resetting of the associatedfilter capacitances. Such a system can instead allow the voltage on theassociated filter capacitance to reach what may be considered a “resetvoltage” for guarding purposes by allowing charge transfer to through apassive impedance for a sufficient amount of time. As another example,some embodiments using oscillators or dual-slope conversions utilizealternating “charging” and “discharging” charge transfer processes toreach upper and lower thresholds, and do not need to be reset at all; insuch cases, either or both the upper or the lower threshold may beconsidered a “reset voltage” for guarding purposes. A third exampleincludes sigma-delta processes for capacitance sensing where the outputof the sigma delta quantizer is kept approximately at a feedbackthreshold, and this feedback threshold may be considered a “resetvoltage” for guarding purposes. These are but a few examples of othersystems that may not actively reset, or even truly reset, but whichstill exhibit what can be considered “reset voltages” for guardingpurposes.

Similarly, pre-determined charging voltages may also change for aparticular sensing system, but the system will still exhibit what can beconsidered a “pre-determined charging voltage” for guarding purposes.For example, embodiments using both “charging” and “discharging” cyclesmay have two or more pre-determined charging voltages producing opposingcharge transfer. In these cases, the “charging” pre-determined chargingvoltage and the “discharging” pre-determined charging voltage can bothbe used to define the guard signal 103.

In various embodiments, the “threshold” voltage is replaced by an A/Dmeasurement of the voltage on the filter capacitance (or representativeof the voltage on the filter capacitance), or by any other voltagedetermination as appropriate. By tracking the number of charge transferiterations and/or the resulting voltage on the filter capacitance(s) asappropriate for the sensing scheme chosen, the amount of chargetransferred to the filter capacitance(s) from the measurablecapacitance(s) can be determined. This amount of charge corresponds tothe value of measurable capacitance(s). Again, alternate embodiments maymake use of other charge transfer schemes, including any sort ofsigma-delta processing whereby the filter capacitance 110 is charged viaa measurable capacitance and discharged by a “delta” charge through animpedance (not shown), or vice versa, and the like.

There are many options for guard signal 103 that would be effective, andfour such options are shown in FIG. 2A by traces 204 (V_(G0)), 205(V_(G1)), 206 (V_(G2)), and 208 (V_(G3)). Trace 204 shows a “sensormatching” option. This “sensor matching” option can be used to match thevoltage on a guarding electrode (e.g. 106) to the expected voltage onthe measurable capacitance (e.g. voltage V_(X) 117A on sensing electrode112A) during the applying the pre-determined voltage steps and thecharge sharing steps of the charge transfer processes of sensorsutilizing switched time constant techniques. Trace 205 shows another“sensor matching” option, which can be used to match the expectedvoltage on the measurable capacitance during the applying thepre-determined voltage steps and the charge sharing steps of the chargetransfer process for systems utilizing switched capacitance techniqueshaving small or negligible time constants. Trace 206 shows a “switchedvoltage divider” option that can be used to approximate the expectedvoltage on the measurable capacitance for each repetition of the chargetransfer process. Trace 208 shows a “pulse coded modulation” signal thatcan be used to approximate the expected voltage on the measurablecapacitance over multiple performances of the charge transfer process.As shown by trace 208, the effect of pulse coded modulation is that theguard voltage of guard signal 103 does not transition with everyperformance of the charge transfer process, but does still follow apattern.

It is understood that multiple types of charge transfer processes may beperformed in synchrony or in series. Multiple similar charge transferprocesses may be used, for example, to determine multiple measurablecapacitances simultaneously or in sequence. Multiple similar chargetransfer processes may also be used concurrently to obtain multipledeterminations of the same measurable capacitance for a more accuratedetermination overall. Charge transfer processes that roughly opposeeach other in effect may also be used to practice more complexmeasurement schemes. For example, a first charge transfer process may beused to charge a filter capacitance and a second charge transfer processmay be used to discharge the same filter capacitance; one or moremeasurement(s) may be taken during the charge and discharge of thefilter capacitance and used to determine the value of the measurablecapacitance. Having such a charge up and charge down scheme may beuseful in reducing the effects of environmental changes.

Multiple types of charge transfer processes (with associated guardvoltages) can also be used to enhance the effects of guarding. Forexample, the pulse coded modulation can be considered to be asuperimposition of multiple types of charge transfer processes (andassociated guard voltages). The pulse coded modulation can thus beconsidered to repeat one, two, or more types of charge transferprocesses (and associated guard voltages) in a particular sequence.These different types of charge transfer processes (and associated guardvoltages) can apply the same predetermined voltage and use the samecomponents, but may involve different guard signals. For example, afirst charge transfer process (and associated guard voltages) caninvolve a first guard voltage and a second guard voltage different fromthe first guard voltage, while a second charge transfer process (andassociated guard voltages) can involve a third guard voltage and afourth guard voltage. In this example, the third guard voltage may bethe same as the first guard voltage or the second guard voltage.Similarly, the fourth guard voltage may be the same as the first guardvoltage or the second guard voltage. Further, the third guard voltageand the fourth guard voltage may be the same or different. The timingand values of the guard voltages would be determined by the averageguard voltage swing appropriate for guarding the applicable sensingelectrodes.

For the embodiment shown in FIGS. 1B-C, the option shown in trace 204(V_(G0)) for guard signal 103 can track the voltage on the measurablecapacitance to help prevent net charge from being gained or lost on thefilter capacitance 110 due to guarded capacitance. Such a “sensormatching” guard signal shown by trace 204 exhibits voltages thatresemble the voltages exhibited by an active sensing electrode (e.g.112A-C) in a sensor using a “switched time constant” sensing techniquesuch as described in FIGS. 1B-C. For example, the guard signal optionshown by trace 204 can be configured to be roughly identical to thevoltage expected for voltage V_(X) 117A of the measurable capacitanceassociated with sensing electrode 112A shown in FIG. 1C (such as byselecting the ratio of capacitance 408 to capacitance 404 in FIG. 4A tobe similar to the ratio of the measurable capacitance associated withsensing electrode 112A to the filter capacitance 110 of FIG. 1B). Thefirst guard voltage of guard signal 103 would approximate that of thecharging pulses 201, while the second guard voltage of guard signal 103would decay to a voltage similar to that on V_(F) 115 with a timeconstant similar to or faster than that exhibited by V_(X) 117A. Thesecond guard voltage of guard signal 103 also varies over executions ofthe charge transfer process, such that it has an overall rise thatapproximates the rise associated with V_(X) 117A during those executionsof the charge transfer process (and the rate of this change in thesecond guard voltage over executions of the charge transfer process canbe considered to be another time constant of the system). A guard signaloption shown by trace 204 can be generated using a circuit similar tothat used by sensor 100 to perform the charge transfer process, or byothers similar to other charge sensing circuitry. Circuits and methodsfor generating this “sensor matching” option by actuating switches totransfer charge onto the applicable guard capacitances are shown inFIGS. 3A-3C, 4A-C and discussed further below.

The option shown in trace 205 for guard signal 103 exhibits morediscrete changes in guarding voltage and lacks the noticeabletime-constant features during a single sharing period associated withthe option shown in by 204. This “switched capacitance” option of trace205 resembles that of a sensing system using a charge transfer processthat actively switches to share the charge between an measurablecapacitance and its associated filter capacitance instead of passivelyallowing charge to share through a passive impedance. The option shownin trace 205 applies a second guard voltage that remains relativelyconstant during a single sharing period but changes over sharingperiods, as would be found in a sensor using a “switched capacitance”type technique for its charge transfer process. Circuits and methods forgenerating this “switched capacitance” option by actuating switches totransfer charge onto the applicable guard capacitances are shown inFIGS. 3C, 4C and discussed further below.

These “sensor matching” options for guard signal 103 may be advantageousover options with “simpler” waveforms (such as those shown in traces 206and 208) in that they can be used to reduce charge transferred to thefilter capacitance(s) due to the guarding electrode for every executionof the charge transfer process, and not just the net charge transferredduring the course of the executions of the charge transfer processesleading to the determination of the measurable capacitance. This isfacilitated by the second guard voltage that changes over repetitions ofthe charge transfer process. However, any guard signal 103 can beeffective if it minimizes the net transfer of charge from the guardingelectrode 106 to the filter capacitance 110 occurring during theexecution of the set of charge transfer processes that eventually resultin the measurement(s) of the voltage on filter capacitance 110 thatis/are used to determine the measurable capacitance. This includes guardsignal options that match a charge transfer process different from theone used by the sensor system, or ones that match no charge transferprocess and simply swing between two or more substantially constantvoltages (discussed below).

In many embodiments, it is often more practical to apply a guard signal103 to guarding electrode 106 that does not minimize charge transferredfrom the guarding electrode 106 to the filter capacitance 110 during asingle execution of the charge transfer process, but does minimize thenet transfer of charge during the set of charge transfer processes thateventually result in measurement(s) of the voltage on filter capacitance110 that are used to determine the applicable measurable capacitance.This can be done with a guard signal 103 that causes charge transfer ina first direction between guarding electrode 106 and filter capacitance110 during one or more executions of the charge transfer process, andcauses charge transfer in a second direction opposite the firstdirection during other execution(s) of the charge transfer process.

As shown by FIG. 2B, charge transferred onto the measurable capacitancefrom the guarding electrode in sharing periods when the voltage on themeasurable capacitance is less than the second guard voltage value 253is effectively restored with charge transferred onto the measurablecapacitance from the guarding electrode in sharing periods when thevoltage on the measurable capacitance is greater than the second guardvoltage value 253. FIG. 2B also shows a guard signal 103 that includes afirst guard voltage 251 for the duration when the pre-determined voltageis applied to the measurable capacitance and a second guard voltage 253for the duration when the measurable capacitances shares. In FIG. 2B,the charge transfer between the guarding electrode 106 and themeasurable capacitance is shown with arrows 230A-G. Arrows 230A-Cindicate periods when charge is transferred from the guarding electrode106 to the measurable capacitance and arrow 203E-G indicate periodswhere charge is transferred to the guarding electrode 106 from themeasurable capacitance. Negligible charge is transferred at arrow (whichappears as a dot) 230D, since voltage 117 is substantially equivalent tothe second guard voltage 253 during that sharing period. The particularvoltage values V_(G) _(—) _(HIGH) 251 and V_(G) _(—) _(LOW) 253 may varysignificantly from embodiment to embodiment. Using this approach, thenet charge transferred to the filter capacitance due to effects of theguarded capacitance can be very small relative to the total charge onthe electrode during the charge transfer process, such that it can beconsidered approximately zero. Balancing the charge transfers betweenthe guarded capacitance and the filter capacitance 110 over a sequenceof executions of charge transfer processes can be further extendedbeyond the examples discussed herein and such extensions are within thescope of this invention.

For example, one option for guard signal 103 would swing between a firstguard voltage approximating the pre-determined voltage and a secondguard voltage approximating the average voltage on filter capacitance110. To determine the average voltage of filter capacitance 110, thevoltage on filter capacitance 110 is averaged over the set of chargetransfer process that leads up to and generates the measurements of thevoltage on filter capacitance 110 used to determine the measurablecapacitance. For a given set of values for the expected measurablecapacitance, filter capacitance, pre-determined voltage, reset voltage,threshold voltage, and ignoring (or accounting for if the model allows)the effects of any passive impedances, well-known methods can be used tomodel the circuit and determine what average filter-capacitance-voltagewould minimize the effect of any guarded capacitances and provide aneffective second guarding voltage. This averagefilter-capacitance-voltage is taken over discrete points, and is roughlythe mean of the voltage on filter capacitance 110 taken over theexecutions of the charge transfer process between the resetting of thefilter capacitance 110 and the last measuring of the filter capacitance110 used to determine the measurable capacitance. Oftentimes, the changein the voltage on filter capacitance 110 will be roughly linear, suchthat the average filter-capacitance-voltage will be approximately themidpoint between the reset voltage and the threshold voltage.

It is also noted that these capacitance sensors are sampled systems(either actually or effectively). For example, in the embodiment shownin FIGS. 1B-C, the filtering capacitance 110 shares charge with themeasurable capacitance only during discrete sharing periods when thepre-determined charging voltage is not applied. In addition, the voltageon the measurable capacitance also usually approaches the voltage 115 onfilter capacitance 110 at the end of the charge sharing period.Therefore, it may be sufficient for the voltages of guard signal 103applied to the guarding electrode 106 to match the voltage on themeasurable capacitance only when the voltage on the measurablecapacitance is “sampled” at the end of the charging period (when theapplying of the pre-determined voltage terminates) and at the end of thecharge sharing period. The end of the charge sharing period occurs whenthe applying of the pre-determined voltage begins in a switchedtime-constant system, such as the one shown in FIGS. 1B-C; the end ofthe charge sharing period occurs when the measurable capacitance isdecoupled from the filter capacitance or when the filter capacitance isdecoupled from any reference voltage, such as in switched capacitancesystems. In other words, if charge sharing occurs through a passivesharing system, technically charge is always being shared; however, forguarding purposes, the charge sharing period may be considered tocontinue only until a subsequent applying of the pre-determined voltage(when charge sharing can be considered to end for guarding purposes). Incontrast, if switching takes place to actively couple and allow sharingof charge by measurable and filter capacitances, the switching may beconsidered to define the end of the charge sharing period.

To that end, the options for guard signal 103 shown by traces 206 and208 can be used. In the “switched voltage divider” option shown by trace206, the actual guard signal 103 may alternate between a first guardvoltage value 251 and a second guard voltage value 253 that approximatesthe “average” value of the voltage 115 on filter capacitance 110.Although this average-V_(F) option has been termed the “switched voltagedivider” option, no voltage divider is required; for example, first andsecond guard voltage values 251 and 253 can be achieved without anyvoltage dividers when they are power supply voltages, are voltagesavailable through a DAC or another part of the sensor, or are producedusing circuitry other than voltage dividers. The “switched voltagedivider” term is used simply because a switched voltage divider circuitwould likely be used in many embodiments of this type of guard signal.In the embodiment described in FIGS. 1A-B, the first guard voltage value251 can be equal to the pre-determined charging voltage and the secondguard voltage value 253 may be approximately equal to the average of athreshold voltage (V_(TH)) used to measure the filter capacitance andthe reset voltage. Circuits and methods for generating this “switchedvoltage divider” are shown in FIGS. 3D-E, 4D-E and discussed furtherbelow.

The timing of the guard signal 103 is based upon the timing of thepulses 201 applied to measurable capacitance in that the guard signal103 has the first guarding voltage value 251 while the charging pulses201 are applied to the measurable capacitance, and the guard signal 103has the second guarding voltage value 253 during the charge sharingperiods between pulses 201. This timing may be useful in that the guardsignal 103 can be driven by existing clocks in the system. However, inpractice, the guard signal 103 can be as effective even if it onlybegins applying the first guarding voltage value 251 sometime after theassociated charging pulse 201 begins, as long as the first guardingvoltage value begins to be applied before the end of the associatedcharging pulse 201. Similarly, the guard signal 103 can be as effectiveeven if it does not apply second guarding voltage value 253 for theentire sharing period, as long as it begins to apply this secondguarding voltage value 253 before the end of the charge sharing period.The timing of the guard signal 206 may not be exactly matched to thecharging pulses 201 for many reasons. For example, imprecise timing maycause the guard signal 103 to start changing to a second guardingvoltage before charge sharing between the measurable capacitance and thefilter capacitance begins, such that the guarding is less effective; toreduce the effects of such imprecise timing, it may be desirable toextend portions of the guard signal 103.

Trace 208 shows an alternate embodiment for guard signal 103 which canbe achieved with fewer components. For example, a single I/O with noadditional components can be used to generate trace 208, as shown inFIG. 4F. With the option shown in trace 208, instead of applying a firstguard voltage value 251 for each of the charging pulses 201, one or morechanges of the guard signal 103 to second guard voltage value 253 can beomitted to adjust the average swing of the guard voltage applied andminimize the net charge transferred by the guarded capacitance to thefilter capacitance (e.g. 110). That is, by extending the duration ofapplication of one guard voltage value (such as the second guard voltagevalue 253 in trace 208) instead of transitioning to the other guardvoltage value (such as to the first guard voltage value 251), theaverage guard voltage swing applied to guarding electrode 106 can bemodified in a manner similar to pulse-coded modulation (also“pulse-frequency modulation”). That is, by applying less frequentcharging pulses (e.g. extending voltage 251 and/or voltage 253) andhaving fewer transitions, the average swing of guard voltage 103 onguarding electrode 106 is reduced, as compared to when more frequentcharging pulses are applied. Notice that the average guard voltage swingcan remain ratiometric to the pre-determined charging voltage swing overmultiple cycles, so as to maintain high performance by improving powersupply noise rejection. Circuits and methods for generating this“pulse-coded modulation” option are shown in FIGS. 3D-E, 4D-E anddiscussed further below.

Many changes can be made to the basic structures and operations shown inFIGS. 1B-C. The timing scheme 150 shown in FIG. 1C assumes a “positive”transfer of charge from the measurable capacitance associated withsensing electrode 112A to filter capacitance 110, for example, whereasequivalent embodiments could be based upon sharing of charge in theopposite direction (that is, positive charge could be placed on filtercapacitance 110 that is drawn through impedance 108 to the measurablecapacitance associated with sensing electrode 112A, then discharged bypulses 201 provided by switch(es) 114). Alternatively, thethreshold-based sensing scheme shown in FIG. 1C could be replaced withany sort of measurement scheme, including any technique based uponmeasurement of the voltage 115 V_(F) on filter capacitance 110 after apre-determined number of executions of the charge transfer process.Further, pulses 201 used to charge or discharge the measurablecapacitance need not be equally spaced in time or be of equal duration.Indeed, in many embodiments, controller 102 could process interrupts orother distractions at virtually any point of the measurement process,since variations in timing are easily tolerated by many of theembodiments shown herein. This is especially true when the sampling timeexceeds the time constants for settling. Alternately, intentionallyvarying the spacing in time of pulses 201 may spread the samplingspectrum to better tolerate noise.

Many changes can be made to the basic structures and operations shown inFIGS. 2A-B. The timing scheme 200 shown in FIG. 2A shows the first guardvoltage is roughly constant and the second guard voltage as the onechanging if such change were to occur. However, since the guard voltage“swing” (difference between the first and second guard voltage asidefrom transition periods) matters more than the actual guard voltagevalues, the guard signal 103 can also be implemented with the firstguard voltage changing instead of the second guard voltage, or bothfirst and second guard voltages changing. Similarly, as discussedearlier, the timing for the guard voltage changes have greatflexibility.

Turning now to FIGS. 3A-E, various embodiments (circuits 104A-E) of aguard voltage generating circuit 104 are shown. Circuit 104 can includeany number of impedances and switches and utilize any number ofreference sources as appropriate. For example, each of the impedancesshown in FIGS. 3A-D can represent the impedance due to a singlecomponent or network of components. Active components in addition toswitches, such as multiplexers, DACs, current sources, or OP-AMPS, canalso be included in guard voltage generating circuit 104, but are notrequired and not used in most embodiments. In addition, the switches ofguard voltage generating circuit 104 can be any discrete switch orrelay, for example, or could correspond to any switching or multiplexingfunctionality contained within controller 102 described above. Switchesused by voltage generating circuit 104 could be implemented asswitch(es) 114 using an I/O pin of controller 102. The output of one I/Ocan sometimes provide multiple switches; for example, a digital I/Ocapable of providing power supply voltages and a high impedance statemay be used to provide the functionality of one multi-way switch, or twoswitches, coupled to one node. Digital I/Os may also provide pull-upresistances, or pull-down resistances or current sources.

If any of the switches are enabled with an I/O capable of providingswitching and measuring functionality, then the sensing system wouldhave the added option of reading the guard signal 103. This would allowthe system to adjust the guard signal 103 dynamically in response towhat voltages it reads as provided to guard signal 103 (such as bychanging the pulse coding if a pulse coded scheme is available).

Impedances of circuit 104 can be any conventional resistances,inductances, capacitances and/or other impedance elements. Thus, thevoltage across an impedance in circuit 104 may be affected by priorhistory of the nodes connected to the impedance. This “prior history”effect may be especially significant for capacitive and inductiveelements, and this effect can be controlled to define the guard signal103. Any reference sources providing references such as referencevoltage can be internal or external to controller 102. Convenientreferences can be used. For example, a reference voltage may be providedby a power supply voltage (V_(dd), GND, −V_(dd)) or battery voltage, andthe like, and the actual reference voltage used may be directly from thesource or some version of these voltages adjusted by impedances. In theexamples shown in FIGS. 3A-E, one reference voltage is shown asreference voltage 301 and a second reference voltage is shown as localsystem ground for convenience of explanation; as discussed earlier,other reference voltage values can be readily used by guard signalgenerating circuit 104.

FIG. 3A shows a configuration of a guard voltage generating circuit 104Aincluding a passive guarding network comprising three impedances 304,306, 308. The three impedances 304, 306, 306 are located in seriesbetween a reference voltage 301 and ground. Switch 302 is located inparallel with impedance 304 and switch 303 is located in parallel withimpedance 308. (As discussed earlier, switches 302 and 303 can beimplemented as switches 114 of FIG. 1 as appropriate) In the embodimentshown in FIG. 3A, guard signal 103 can be provided by appropriatelyswitching switches 302, 303. When switch 302 is closed and switch 303 isopen, the voltage of guard signal 103 is determined by the referencevoltage 301 and the voltages across impedances 306 and 308. This voltageof guard signal 103 could correspond to a reset voltage of a chargetransfer process that is being guarded. When both switches 302 and 303are open, the voltage of guard signal 103 is determined by the referencevoltage 301 and the voltages across impedances 304, 306, and 308. Thisvoltage of guard signal 103 could correspond to the voltage on a filtercapacitance in a charge transfer process that is being guarded. Whenswitch 302 is open and switch 303 is closed, the voltage of guard signal103 is driven to GND. This voltage of guard signal 103 could correspondto a pre-determined charging voltage of a charge transfer process thatis being guarded. With proper choice of impedances 304, 306, 308, aconfiguration such as circuit 104A allows a guard voltage generatingcircuit that emulates the voltages associated with charge transferprocesses utilizing a “switched time constant” technique, such as inFIGS. 1B-C. For example, the impedance 304 could be configured tocorrespond with a filter capacitance formed from a network ofcomponents, and impedance 304 could be coupled to more than one voltageto accurately correspond to that of the matched filter capacitance. Notethat a variety of reset voltages and charging voltages may be guardedthough they may require different switching sequences or referencesvoltages (e.g. V_(dd) and ground).

For the embodiment shown in FIG. 3A, when switch 302 is open and switch303 is open, impedances 304, 306, and 308 form an impedance divider with“common nodes” where impedance 306 connects to impedance 308 and whereimpedance 304 connects to impedance 306. When switch 302 is closed andswitch 303 is open, impedances 306 and 308 form a different impedancedivider with a common node where impedance 306 connects to impedance308.

An impedance divider is composed of at least two passive impedances inseries, where each passive impedance is coupled to at least two nodes.One of these nodes is common to both impedances (“a common node” towhich both impedances connect.) The common node serves as the output ofthe impedance divider. The output of the impedance divider is a functionof the voltages and/or currents applied at the “unshared” nodes (thenodes of the two impedances that are not the common node) over time. Asimple example of an impedance divider is a voltage divider composed oftwo capacitances or two resistances. More complex impedance dividers mayhave unmatched capacitances, resistances, or inductances in series or inparallel. One impedance may also have any combination of capacitive,resistive, and inductive characteristics.

In the exemplary embodiment of guard voltage generating circuit 104Bshown in FIG. 3B, the passive guarding network is comprised of impedance314. For circuit 104B, guard signal 103 is suitably switched by switch312 between reference voltage 301 when switch 312 is closed; thisvoltage of guard signal 103 could correspond to a pre-determinedcharging voltage. Guard signal 103 suitably switches to a second voltagedefined by the voltage across impedance 314 when switch 312 is open;this voltage of guard signal 103 could correspond to the voltage on afilter capacitance. Switch 313 could be closed to remove charge fromimpedance 314; this voltage of guard signal 103 can correspond to areset voltage. With proper choice of impedance 314, a configuration ascircuit 104B allows a guard voltage generating circuit that emulates thevoltages associated with a charge transfer processes utilizing asigma-delta version of the “switched time constant” technique.

FIG. 3C shows another embodiment of the guard voltage generating circuit104C that includes a passive guarding network comprised of twoimpedances 324, 326 in series. Circuit 104C is driven by three switches322, 323, and 325. When switch 322 is closed and switches 323 and 325are open, the guard signal 103 is the reference voltage 301; thisvoltage of guard signal 103 could correspond to a pre-determinedcharging voltage. When switches 322 and 323 are open, and switch 325 isclosed, the guard signal 103 is determined by the reference voltage 301and the voltage across impedances 324, 326; this voltage of guard signal103 could correspond to the voltage on a filter capacitance. When switch323 and 325 are closed and switch 322 is open, the guard signal 103 isGND and the charge on impedance 326 is removed; this voltage of guardsignal 103 could correspond to a reset voltage. When switches 322 and323 are open and switch 325 is closed, the impedances 324 and 326 forman impedance divider with a common node at the guard signal 103 output.With proper choice of impedances 324 and 326, a configuration such ascircuit 104C allows a guard voltage generating circuit that emulates thevoltages associated with charge transfer processes utilizing a “switchedcapacitance” technique.

FIG. 3D shows an embodiment of the guard voltage generating circuit 104Dwith a passive guarding network comprising two impedances 334 and 336located in series with the reference voltage 301 and a switch 332 toground (GND). In circuit 104D, guard signal 103 is suitably switchedusing switch 332. When switch 332 is open, the guard signal 103 isdetermined by reference voltage 301 and the voltage across impedance334; this voltage of guard signal 103 could correspond to apre-determined voltage. When switch 332 is closed, the guard signal 103is determined by reference voltage 301 and the voltages acrossimpedances 334 and 336; this voltage of guard signal 103 couldcorrespond to an average voltage on a filter capacitance. When switch332 is closed, the impedances 334 and 336 form an impedance divider thatappropriately divides the reference voltage 301 as determined by thetype and value of impedance components chosen. That is, impedances 334and 336 suitably function as a “pull-up” component when switch 332 isopen, and impedances 334 and 336 function as an impedance divider whenswitch 302 is closed. In the simple case where resistors are used forimpedances 334 and 336, the impedance divider is a conventional voltagedivider and the guard signal 103 when switch 332 closed is proportionalto reference voltage 301 via the ratio of the resistance of impedance336 to the sum of the resistances of impedances 334 and 336. With properchoice of impedances 324 and 326, a configuration as circuit 104D allowsa guard voltage generating circuit 104 for “switched voltage divider”type of guard signal 103. The output of circuit 104D can be furtheradapted, such as modulated in frequency, to produce a “pulse codedmodulation” type of waveform for guard signal 103.

FIG. 3E shows another embodiment of guard signal generating circuit 104Ethat includes a two switches 342 and 343 coupled to reference voltage301 and ground, respectively, and no discrete impedances. In theembodiment of 104E, the passive guarding network can thus comprise asimple wire. In circuit 104E, the guard signal suitably switches betweenreference voltage 301 when switch 342 is closed and switch 343 is open,and ground when switch 342 is open and switch 343 is closed. Theconfiguration of circuit 104E allows a guard voltage generating circuit104 to provide a degenerate “switched voltage divider” type of guardsignal 103 (where there is no voltage divider and the guard signalswitches between undivided reference voltage 301 and ground). Theconfiguration of circuit 104E is especially useful for a “pulse codedmodulation” type of waveform for guard signal 103, where the guardsignal 103 does not change in voltage in step with all repetitions ofthe charge transfer process used to detect stimulus 101.

The embodiments of guard voltage generating circuit 104 shown in FIGS.3A-3E are but five examples of the various alternatives that can be usedto determine the guard signal 103. Many other options for providingguard signal 103 using switches with and without passive guardingnetworks comprised of impedances in series and/or parallel and arecontemplated here. These alternatives may be quite similar to thoseshown in FIGS. 3A-3E. For example, an additional impedance could coupleimpedance 306 to another reference voltage in parallel with impedance304 for circuit 104A. As another example, impedance 314 of circuit 104Bcan be in parallel with switch 312 instead of switch 313. As a thirdexample, switch 325 of circuit 104C can couple impedance 324 toreference voltage 301 instead of couple impedance 326 to ground. As afurther example, switch 332 of 104D can be coupled between impedance 334and reference voltage 301 instead of between impedance 336 and ground.Other alternatives may differ more drastically, and involve impedancesand switches in other configurations.

Turning to FIGS. 4A-E, examples with more detail of guard voltagegenerating circuits 104 are shown in conjunction with a controller suchas the controller 102 of FIG. 1B. The exemplary circuit 104F shown inFIG. 4A, is an embodiment of the circuit 104A shown in FIG. 3A whereimpedance 304 is implemented as a capacitance 404, impedance 306 isimplemented as resistance 406, and impedance 308 is implemented ascapacitance 408 and where switch 302 has been implemented using I/O 402and switch 303 has been implemented using I/O 403. The configuration ofcircuit 104F is quite similar to that of the circuitry used to practicethe charge transfer process of sensor 100 (FIG. 1A). Capacitance 408 isanalogous to a measurable capacitance, resistance 406 is analogous to apassive impedance (e.g. 108A-C), and capacitance 404 is analogous to thefilter capacitance 110. Switch 302 as implemented using I/O 402 isanalogous to switch 118, and switch 303 as implemented using I/O 403 isanalogous to switches 116A-C implemented using I/O 119 (FIG. 1B). I/O403 itself is analogous to I/O 119 (FIG. 1B). The circuit 104F can thusbe driven in a way to match the charge transfer process such that theguard signal 103 would roughly match the voltage 117 of a chargetransfer sensing process as shown in FIGS. 1B-C, and minimize chargetransfer from the guarding electrode 106 to the filter capacitance 110at all points of the charge transfer processes used for sensing. Even ifa guard signal 103 that differs from voltage 117 is generated usingcircuit 104F, it can still be quite effective if it minimizes overallcharge transferred between the guarding electrode 106 and the filtercapacitance 110 for the set of charge transfer processes that results inthe measurements used to determine the value of measurable capacitance.

The example circuit 104G shown in FIG. 4B is an embodiment of thecircuit 104B of FIG. 3B. Both switches 312 and 313 have been implementedusing a single I/O 412, and the impedance 314 has been implemented as anetwork having a resistance 414 and capacitance 415. The example circuit104G can be driven using something similar to a “one I/O sigma delta”type “switched time constant” methodology. In such a methodology, switch313 of I/O 412 is opened (if it is not already open) and switch 312 ofI/O 412 is closed to apply the reference voltage 301 (which is thepre-determined voltage), and then switch 312 of I/O 412 is opened toallow charge to share between any guarded capacitances in the system andcapacitance 415. When switch 312 of I/O 412 is closed, the capacitance415 is charged through impedance 414. Closing switch 313 of I/O 412discharges capacitance 415 through impedance 414. The voltage oncapacitance 415 can be measured using I/O 412, and this voltage can bereduced as necessary by closing switch 313 of I/O 412 when thepre-determined voltage is applied to the measurable capacitance (so asnot to directly affect the guarded capacitance charge transfer). In thisway, the voltage on capacitance 415 can be controlled to the secondguarding voltage. This cycle of first opening switch 313 and closingswitch 312 of I/O 412, and then opening switch 312 and closing switch313 of I/O 412 can be repeated in synchrony with the charge transferprocess used to detect proximity and measure the measurable capacitance.The circuit 104G can thus be driven in a way to generate a guard signal103 that roughly matches the voltages of the measurable capacitance in acharge transfer process such as the one shown in FIGS. 1A-B. Circuit104G can also be driven in a way to generate a guard signal 103 thatclosely matches the voltages of the measurable capacitance in a singleI/O sigma-delta charge transfer process.

The example circuit 104H shown in FIG. 4C is an embodiment of thecircuit 104C of FIG. 3C. Switches 322 and 323 have been implementedusing I/O 422, and switch 325 has been implemented using I/O 425.Impedance 324 has been implemented as capacitance 424, and impedance 326has been implemented as capacitance 426. The example circuit 104H isanalogous to a “switched capacitance” circuit where capacitance 424(which is a fixed capacitance) is analogous to the measurablecapacitance and capacitance 426 is analogous to the filter capacitance.Example circuit 104H can be driven using something similar to a“switched capacitance” methodology. In such a methodology, switch 322 ofI/O 422 is closed and switch 323 of I/O 422 is opened to apply thereference voltage 301 (which is the pre-determined voltage in theembodiment shown in FIG. 4C) to capacitance 424. Then, switch 322 of I/O422 is opened and switch 325 of I/O 425 is closed to allow charge toshare between capacitances 424 and 426. This cycle of first closingswitch 322 of I/O 422 and then opening switch 322 of I/O 422 and closingswitch 325 of I/O 425 can be repeated synchronous with the chargetransfer process used to detect proximity and measure the measurablecapacitance. After the appropriate number of cycles (such as to when thenumber of executions of the charge transfer process used to generate theresults used to determine the measurable capacitance have beenperformed), switch 323 of I/O 422 and switch 325 of I/O 425 can close toreset the charge on capacitance 426. The circuit 104G can thus be drivenin a way to generate a guard signal 103 that has a first guard voltagethat is the pre-determined voltage and a second guard voltage that issubstantially constant within an execution of the charge transferprocess but that rises from the reset voltage with each subsequentexecution of the charge transfer process before reset. This guard signal103 would then approximate the voltages of the measurable capacitance ina charge transfer process if the ratio of the fixed capacitance 424 tocapacitance 426 is comparable to the ratio of the measurable capacitanceto the filter capacitance.

The example guard signal generating circuit 1041 shown in FIG. 4D is anembodiment of the circuit 104D shown in FIG. 3D. Impedance 334 has beenimplemented using resistance 434, impedance 336 has been implementedusing resistance 436, and switch 332 has been implemented using I/O 432.When switch 332 of I/O 432 is open, the guard signal 103 approaches thereference voltage 301. When switch 332 of I/O 432 is closed, the guardsignal 103 is set to a voltage that is proportional to the referencevoltage 301 by the ratio of resistance 436 to the sum of resistances 434and 436. With the circuit 1041 embodiment, a guard signal 103 can beused to approximate the average swing of voltage associated with themeasurable capacitance. For example, for the sensor 100 of FIG. 1B, thefirst guard voltage can be applied by opening switch 332 of I/O 432 andapplying reference voltage 301 (which can be the pre-determined voltage,for example). Then, the second guard voltage can be applied by closingswitch 332 of I/O 432 and applying a fraction of reference voltage 301(which can be halfway between the applicable threshold voltage and thereset voltage, for example). With the proper timing of the first andsecond guard voltages defining when and how long they are appliedrelative to each other and the steps of the charge transfer process usedfor sensing, and with the proper selection of resistance and referencevoltage values, this guard signal 103 can then exhibit a voltage swingthat would then approximate the average voltage swing of the applicablemeasurable capacitance in the charge transfer executions and provideeffective guarding.

The signal 103 of circuit 1041 can be further adapted with pulse codedmodulation of the switching of switch 332. By changing the frequency ofthe switching and thus the transition between the guard voltages, adifferent actual guard voltage swing can be generated. Pulse codedmodulation can actually be applied to any circuit 104 when control ofthe frequency of transition is available. However, in cases where theguard signal 103 already approximates the actual voltage 117 exhibitedby the measurable capacitance or its average, pulse coding may offerlittle or no advantage.

The example guard signal generating circuit 104J shown in FIG. 4E is anembodiment of the circuit 104E shown in FIG. 3E. The I/O 442 can bedirectly connected to the guarding electrode(s), such that there isnegligible impedance. Switches 344 and 346 of circuit 104E have beenimplemented using a single I/O 442. When switch 342 of I/O 442 is closedand switch 343 of I/O 442 is open, the guard signal 103 is set to thelogic “high” reference voltage 301 (e.g. V_(dd) if I/O 442 is aconventional digital I/O). When switch 342 of I/O 442 is open and switch343 of I/O 442 is closed, the guard signal 103 is set to logic “low”reference voltage (e.g. ground). With the circuit 104J embodiment, sincethe reference voltage and ground may be set by the limitations ofcontroller 102, it is likely more difficult to generate a guard signal103 with a swing for each charge transfer process. Therefore, thecircuit 104J may be very amenable to pulse coded modulation. With theproper ratio of transitions between the first and second guard voltages(which can be the pre-determined voltage and ground, respectively), anaverage guard voltage swing can be generated for guard signal 103 thatapproximates the average voltage swing exhibited by the measurablecapacitance. For example, if the guard signal 103 transitions betweenthe first and second guard voltages three times for every fiveexecutions of the charge transfer process for detecting proximity, theaverage guard voltage swing is three-fifth of the voltage swing betweenone transition of the first and second guard voltages.

As discussed earlier, in all of the examples 4A-4E where the switchingis generated using a component that also has measurement capabilities,such as using a digital I/O of a controller, the I/O can also be used tomeasure the voltage of guard signal 103 as to adjust the guard signal103 as necessary. The adjustment may take place for the current set ofexecutions of charge transfer processes used to generate themeasurement(s) for determining the measurable capacitance, or may takeplace for the next set of charge transfer processes.

As noted above, many of the embodiments described herein may be readilyimplemented using commercially-available components such as conventionalintegrated circuits and any combination of discrete resistors and/orcapacitors. Because of this simplicity, many different types of sensors100 can be created that share or do not share various components and/orswitches. For example, the measurable capacitances associated with thesensing electrodes 112A-C in FIG. 1B are coupled to a common filtercapacitance 110, but in practice each channel could be coupled to itsown filter capacitance 110. Similarly, one or more passive impedances108A-C and/or any number of switches (e.g. 114, 116A-C, 118) and I/Os(e.g. I/O 119) could be shared between sensing channels in alternateembodiments. This sharing may be exploited across many additionalchannels to create sensors capable of efficiently sensing numerousmeasurable capacitances with a single controller 102. This sharing canreduce cost and size of the overall sensor 100 significantly.

By implementing multiple sensing channels on a common controller 102, anumber of efficiencies can be realized. Frequently, sensing electrodesand/or guarding electrode(s) can be readily formed on a standard printedcircuit board (PCB), so duplication of these elements is relativelyinexpensive in a manufacturing sense. In a case where the measurablecapacitances are expected to be relatively small, then filtercapacitance 110 may also be manufacturable in a PCB. In addition, noneor one or more resistances, capacitances, and inductances may be formedon a PCB to provide impedances used in the guard voltage generatingcircuit 104, such as capacitance 404 and resistance 406 of circuit 104F.As a result, many of the various features described above can be readilyimplemented using conventional manufacturing techniques and structures.However, in some cases, components such as filter capacitance(s) and/orpassive impedance(s) and other impedances may be large enough or requiretight enough tolerances to warrant discrete components in manyembodiments. In those cases, these components (e.g. filter capacitance110) may be implemented with one or more discrete capacitors, resistors,inductors, and/or other discrete components.

Moreover, the total number of signal pins (e.g. those of ADCs and I/Os)required and the number of components can be even further reducedthrough use of time, frequency, encoding or other multiplexingtechnique.

Arranging the sensing electrodes 112A-B in any number of patterns alsoallows for many diverse types of sensor layouts (includingmulti-dimensional layouts found in touchpads capable of sensing in one,two or more-dimensions) to be formulated. Alternatively, multiple“button”-type touch sensors and combinations of button-type andtouchpad-type input devices can be readily formed from the variouschannels, or any number of other sensor layouts could be created.

As stated above, the devices and methods for determining capacitance areparticularly applicable for use in proximity sensor devices. Turning nowto FIG. 5, a block diagram is illustrated of an exemplary electronicsystem 10 that is coupled to a proximity sensor device 11. Electronicsystem 10 is meant to represent any type of personal computer, portablecomputer, workstation, personal digital assistant, video game player,communication device (including wireless phones and messaging devices),media device, including recorders and players (including televisions,cable boxes, music players, and video players) or other device capableof accepting input from a user and of processing information.Accordingly, the various embodiments of system 10 may include any typeof processor, memory or display. Additionally, the elements of system 10may communicate via a bus, network or other wired or wirelessinterconnection. The proximity sensor device 11 can be connected to thesystem 10 through any type of interface or connection, including I2C,SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, IRDA, or any othertype of wired or wireless connection to list several non-limitingexamples.

Proximity sensor device 11 includes a controller 19 and a sensing region18. Proximity sensor device 11 is sensitive to the position of a stylus114, finger and/or other input object within the sensing region 18 bymeasuring the resulting capacitance. “Sensing region” 18 as used hereinis intended to broadly encompass any space above, around, in and/or nearthe proximity sensor device 11 wherein the sensor is able to detect aposition of the object. In a conventional embodiment, sensing region 18extends from the surface of the sensor in one or more directions for adistance into space until signal-to-noise ratios prevent objectdetection. This distance may be on the order of less than a millimeter,millimeters, centimeters, or more, and may vary significantly with thesize of sensing electrodes, type of position sensing technology used,and the accuracy desired. Accordingly, the planarity, size, shape andexact locations of the particular sensing regions 18 will vary widelyfrom embodiment to embodiment.

In operation, proximity sensor device 11 suitably detects a position ofstylus 14 by measuring the measurable capacitance associated with theplurality of electrodes and finger or other input object within sensingregion 18, and using controller 9, provides electrical or electronicindicia of the position to the electronic system 10. The system 10appropriately processes the indicia to accept inputs from the user, tomove a cursor or other object on a display, or for any other purpose.

In a common implementation of a touch sensor device a voltage istypically applied to create an electric field across a sensing surface.A capacitive proximity sensor device 11 would then detect the positionof an object by detecting changes in capacitance caused by the changesin the electric field due to the object. For example, the sensor ofproximity sensor device 11 can use arrays of capacitive sensingelectrodes to support any number of sensing regions. As another example,the sensor can use capacitive sensing technology in combination withresistive sensing technology to support the same sensing region ordifferent sensing regions. Depending on sensing technique used fordetecting object motion, the size and shape of the sensing region, thedesired performance, the expected operating conditions, and the like,proximity sensor device 11 can be implemented with a variety ofdifferent ways. The sensing technology can also vary in the type ofinformation provided, such as to provide “one-dimensional” positioninformation (e.g. along a sensing region) as a scalar, “two-dimensional”position information (e.g. horizontal/vertical axes, angular/radial, orany other axes that span the two dimensions) as a combination of values,and the like.

The controller 19, sometimes referred to as a proximity sensor processoror touch sensor controller, is coupled to the sensor and the electronicsystem 10. In general, the controller 19 measures the capacitance usingany of the various techniques described above, and communicates with theelectronic system. The controller 19 can perform a variety of additionalprocesses on the signals received from the sensor to implement theproximity sensor device 11. For example, the controller 19 can select orconnect individual sensing electrodes, detect presence/proximity,calculate position or motion information, and report a position ormotion when a threshold is reached, and/or interpret and wait for avalid tap/stroke/character/button/gesture sequence before reporting itto the electronic system 10, or indicating it to the user. Thecontroller 19 can also determine when certain types or combinations ofobject motions occur proximate the sensor.

In this specification, the term “controller” is defined to include oneor more processing elements that are adapted to perform the recitedoperations. Thus, the controller 19 can comprise all or part of one ormore integrated circuits, firmware code, and/or software code thatreceive electrical signals from the sensor, measure capacitance of theelectrodes on the sensor, and communicate with the electronic system 10.In some embodiments, the elements that comprise the controller 19 wouldbe located with or near the sensor. In other embodiments, some elementsof the controller 19 would be with the sensor and other elements of thecontroller 19 would reside on or near the electronic system 100. In thisembodiment minimal processing could be performed near the sensor, withthe majority of the processing performed on the electronic system 10.

Again, as the term is used in this application, the term “electronicsystem” broadly refers to any type of device that communicates withproximity sensor device 11. The electronic system 10 could thus compriseany type of device or devices in which a touch sensor device can beimplemented in or coupled to. The proximity sensor device could beimplemented as part of the electronic system 10, or coupled to theelectronic system using any suitable technique. As non-limiting examplesthe electronic system 10 could thus comprise any type of computingdevice, media player, communication device, or another input device(such as another touch sensor device or keypad). In some cases theelectronic system 10 is itself a peripheral to a larger system. Forexample, the electronic system 10 could be a data input or outputdevice, such as a remote control or display device, that communicateswith a computer or media system (e.g., remote control for television)using a suitable wired or wireless technique. It should also be notedthat the various elements (processor, memory, etc.) of the electronicsystem 10 could be implemented as part of an overall system, as part ofthe touch sensor device, or as a combination thereof. Additionally, theelectronic system 10 could be a host or a slave to the proximity sensordevice 11.

It should be noted that although the various embodiments describedherein are referred to as “proximity sensor devices”, “touch sensordevices”, “proximity sensors”, or “touch pads”, these terms as usedherein are intended to encompass not only conventional proximity sensordevices, but also a broad range of equivalent devices that are capableof detecting the position of a one or more fingers, pointers, styliand/or other objects. Such devices may include, without limitation,touch screens, touch pads, touch tablets, biometric authenticationdevices, handwriting or character recognition devices, and the like.Similarly, the terms “position” or “object position” as used herein areintended to broadly encompass absolute and relative positionalinformation, and also other types of spatial-domain information such asvelocity, acceleration, and the like, including measurement of motion inone or more directions. Various forms of positional information may alsoinclude time history components, as in the case of gesture recognitionand the like. Accordingly, proximity sensor devices can appropriatelydetect more than the mere presence or absence of an object and mayencompass a broad range of equivalents.

It should also be understood that while the embodiments of the inventionare described herein the context of a fully functioning proximity sensordevice, the mechanisms of the present invention are capable of beingdistributed as a program product in a variety of forms. For example, themechanisms of the present invention can be implemented and distributedas a proximity sensor program on a computer-readable signal bearingmedia. Additionally, the embodiments of the present invention applyequally regardless of the particular type of signal bearing media usedto carry out the distribution. Examples of signal bearing media include:recordable media such as memory cards, optical and magnetic disks, harddrives, and transmission media such as digital and analog communicationlinks.

Various other modifications and enhancements may be performed on thestructures and techniques set forth herein without departing from theirbasic teachings. Accordingly, there are provided numerous systems,devices and processes for detecting and/or quantifying one or moremeasurable capacitances. While at least one exemplary embodiment hasbeen presented in the foregoing detailed description, it should beappreciated that a vast number of variations exist. The various steps ofthe techniques described herein, for example, may be practiced in anytemporal order, and are not limited to the order presented and/orclaimed herein. It should also be appreciated that the exemplaryembodiments described herein are only examples, and are not intended tolimit the scope, applicability, or configuration of the invention in anyway. Various changes can therefore be made in the function andarrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A device for determining a measurable capacitance for proximitydetection, the device comprising: a plurality of sensing electrodes; aguarding electrode proximate the plurality of sensing electrodes; apassive network that includes a filter capacitance; and a controllercoupled to the plurality of sensing electrodes, the guarding electrode,and the passive network, the controller configured to apply apre-determined voltage to at least one of the plurality of sensingelectrodes using a first switch, apply a first guard voltage to the atleast one guarding electrode using a second switch, share charge betweenat least one of the plurality of sensing electrodes and the passivenetwork such that shared charge is accumulated on the filtercapacitance, apply a second guard voltage different from the first guardvoltage to the guarding electrode; and measure a voltage on the filtercapacitance for a number of measurements equal to at least one toproduce at least one result to determine the measurable capacitance forproximity detection.
 2. The device of claim 1 wherein the plurality ofsensing electrodes and the guarding electrode are formed as circuits ofa printed circuit board.
 3. The device of claim 1 wherein the firstguard voltage comprises a voltage selected to approximate thepredetermined voltage applied to the at least one of the plurality ofsensing electrodes.
 4. The device of claim 1 wherein the second guardvoltage comprises a voltage selected to be in a range between andincluding a threshold voltage and a reset voltage.
 5. The device ofclaim 1 wherein the second guard voltage comprises a variable voltagewith a time constant selected to approximate voltage on the filtercapacitance.
 6. The device of claim 1 wherein the wherein the secondguard voltage comprises a pulse modulated signal.
 7. The device of claim1 wherein the second guard voltage comprises a voltage approximating apredetermined average voltage on the filter capacitance.
 8. The deviceof claim 1 wherein the plurality of electrodes includes at least onetransmitting electrode and at least one receiving electrode, and whereinthe measurable capacitance comprises a transcapacitance establishedbetween the at least one transmitting electrode and the at least onereceiving electrode.
 9. The device of claim 1 wherein the controller isfurther configured to share charge using a third switch.
 10. The deviceof claim 1 wherein the controller is further configured to apply thesecond guard voltage using a third switch.
 11. A device for measuring acapacitance value, the device comprising: a passive network coupled to afirst capacitance, wherein the passive network is configured to storecharge received from the first capacitance; a charge changing circuitcoupled to the passive network; a guarding electrode proximate the firstcapacitance; a controller configured to measure the capacitance value byrepeatedly applying a predetermined voltage to the first capacitance,repeatedly storing charge received from the first capacitance on thepassive network, and repeatedly comparing a voltage on the passivenetwork to a threshold of a quantizer and changing a charge on thepassive network by a quantized amount of charge using the chargechanging circuit in response to the voltage on the passive network beingpast the threshold of the quantizer, the quantized amount of chargedetermined at least in part as a function of an output of the quantizer;and wherein the controller is configured to apply a guarding signal tothe guarding electrode, the guarding signal including at least a firstguard voltage level and a second guard voltage level.
 12. The device ofclaim 11 wherein the first capacitance is formed in part from a first ofa plurality of sensing electrodes, and wherein the plurality of sensingelectrodes and the guarding electrode are formed as circuits of aprinted circuit board.
 13. The device of claim 11 wherein the firstguard voltage level comprises a voltage selected to approximate thepredetermined voltage applied to the first capacitance.
 14. The deviceof claim 11 wherein the second guard voltage level comprises a voltageselected to be outside of a range between and including a thresholdvoltage and the predetermined voltage.
 15. The device of claim 11wherein the wherein the second guard voltage comprises a pulse modulatedsignal.
 16. The device of claim 11 wherein the second guard voltagecomprises a voltage approximating a predetermined average voltage on thepassive network.
 17. The device of claim 11 wherein the second guardvoltage level comprises a voltage approximating a threshold voltage ofthe quantizer.
 18. A device for measuring a measurable capacitance, thedevice comprising: a filter capacitance; a passive impedance staticallycoupling the filter capacitance to the measurable capacitance; aguarding electrode proximate the measurable capacitance; and acontroller coupled to the measurable capacitance, wherein the controlleris configured to perform a charge transfer process, wherein the chargetransfer process comprises applying a predetermined voltage to themeasurable capacitance and allowing the measurable capacitance to sharecharge with the filter capacitance through the passive impedance, andwherein the controller is further configured to determine a value of themeasurable capacitance as a function of an accumulated charge on thefilter capacitance; and wherein the controller is configured to apply aguarding signal to the guarding electrode, the guarding signal includingat least a first guard voltage level and a second guard voltage level.19. The device of claim 18 wherein the measurable capacitance is formedin part from a first of a plurality of sensing electrodes, and whereinthe plurality of sensing electrodes and the guarding electrode areformed as a circuit of a printed circuit board.
 20. The device of claim18 wherein the first guard voltage comprises a voltage selected toreduce the charge accumulated from the guarding electrode through thesensing electrode and onto the filter capacitance.
 21. The device ofclaim 18 wherein the second guard voltage comprises a voltage selectedto be in a range between and including a threshold voltage and a resetvoltage.
 22. The device of claim 18 wherein the second guard voltagecomprises a variable voltage with a time constant selected toapproximate voltage on the filter capacitance.
 23. The device of claim18 wherein the wherein the second guard voltage comprises a pulsemodulated signal.
 24. The device of claim 18 wherein the second guardvoltage comprises a voltage approximating a predetermined averagevoltage on the filter capacitance.
 25. The device of claim 18 whereinthe controller is further configured to reset a voltage on the filtercapacitance using a switch.